MS HVAC cheat sheet

Sensible heat formula: Qs = 1.08 × CFM × ΔT (BTU/h, standard air at sea level); latent heat: QL = 0.68 × CFM × ΔW (grains/lb).

Total heat: QT = 4.5 × CFM × Δh (enthalpy difference, BTU/lb dry air), which equals sensible plus latent load.

Standard air density is 0.075 lb/ft³ at 70°F and sea level; at 5000 ft altitude use roughly 0.062 lb/ft³ and derate capacity about 15%.

Pressure conversions: 1 psi = 2.31 ft of water column = 27.7 in. w.c.; 1 in. w.c. ≈ 0.036 psi ≈ 249 Pa.

Superheat = actual suction line temp minus saturation temp at suction pressure; subcooling = saturation temp at liquid pressure minus liquid line temp.

Refrigeration cycle order: compressor → condenser → metering device → evaporator → back to compressor; heat is absorbed in the evaporator and rejected in the condenser.

1 ton of refrigeration = 12,000 BTU/h = 200 BTU/min = 3.517 kW; 1 kW = 3,412 BTU/h.

Sensible heat factor (SHF) = sensible load ÷ total load; a room SHF near 1.0 indicates mostly sensible load with little moisture removal.

Temperature scales: °F = (°C × 1.8) + 32; absolute Rankine = °F + 460; Kelvin = °C + 273.

Compressor types by application: reciprocating and scroll for residential/light commercial; screw and centrifugal for large tonnage and chillers.

Scroll compressors are unidirectional; running one backward (phase reversal) damages the scroll set, so verify phase rotation on three-phase units.

TXV controls evaporator superheat (typically 8–12°F at the evaporator outlet); a fixed orifice or piston meters by pressure difference and is charge-sensitive.

Low superheat at the compressor with a cold, sweating suction line suggests overcharge or a stuck-open TXV; high superheat with low suction pressure suggests undercharge or restriction.

Subcooling target on a TXV system is typically 8–12°F at the condenser outlet; low subcooling with bubbles in the sight glass indicates undercharge.

Low-side (suction) pressure corresponds to evaporator saturation temperature; high-side (discharge) pressure corresponds to condensing saturation temperature.

Liquid line filter-driers remove moisture and acid; suction line filter-driers are used temporarily after a burnout and are removed afterward.

High discharge pressure causes: dirty condenser, non-condensable air, overcharge, low condenser airflow, or high ambient temperature.

Evacuation to 500 microns with a standing vacuum test confirms removal of moisture and non-condensables before charging.

Ohm's law: E = I × R; power: P = E × I (watts); for single-phase use P = E × I × PF when power factor applies.

Capacitor check: a good run capacitor reads within its rated µF tolerance; a bulging or open capacitor prevents compressor or fan start.

PSC motors use a run capacitor; a start capacitor with a potential relay is added for hard-start conditions and is switched out after startup.

Contactors and relays: the coil is the control circuit, the contacts are the load circuit; a pitted contactor causes voltage drop and compressor overheating.

Control transformer steps line voltage down to 24 VAC for thermostats, gas valves, and contactor coils; a blown low-voltage fuse usually means a shorted control wire or component.

Thermostat wiring convention: R = 24V power, W = heat, Y = cooling compressor, G = indoor fan, O/B = reversing valve, C = common.

Locked rotor amps (LRA) is the inrush current at startup; running load amps (RLA) is the normal operating current; a compressor drawing near LRA while running indicates a seized or shorted winding.

Sequence of operation: thermostat calls for cooling → Y and G energize → contactor closes → compressor and condenser fan start → indoor blower runs → safety switches (high-pressure, low-pressure, freeze stat) can interrupt the circuit.

Electrical safety: lock out and tag out, verify zero voltage with a meter, and discharge capacitors before servicing.

Duct sizing: equal friction method uses a constant pressure drop per 100 ft (typically 0.1 in. w.c./100 ft); velocity method sets a target velocity per duct type.

Return air and supply air must be balanced; a static pressure above about 0.5 in. w.c. at the blower outlet suggests restricted ductwork or a dirty filter.

Friction loss in round duct rises with the square of velocity; doubling airflow through the same duct roughly quadruples friction loss.

Equivalent length: each fitting (elbow, tee, transition) adds equivalent feet of straight duct; sharp 90° elbows cost far more than long-radius or turning-vane elbows.

Airflow measurement: CFM = velocity (ft/min) × free area (ft²); a duct traverse with a pitot tube or anemometer gives average velocity.

Duct leakage and infiltration raise the load; sealing joints, mastic, and proper insulation reduce energy loss and condensation on cold ducts.

Indoor air quality controls: MERV 8–13 filters for particulate, UV or PCO for biologicals, and ventilation per ASHRAE 62.1 for outdoor air.

A dirty evaporator coil or filter reduces airflow, lowers suction pressure, and can cause coil icing; a restricted return raises the static pressure and starves the blower.

Gas furnace sequence: thermostat calls for heat → inducer draft motor starts → pressure switch proves draft → igniter heats → gas valve opens → flame sensor proves flame → blower starts after a delay.

Combustion air: natural gas requires about 10 ft³ of air per 1,000 BTU/h of input; a starved burner causes rollout, soot, or flame rollout switch trips.

Furnace efficiency: AFUE is annual fuel utilization efficiency; 80% AFUE units are non-condensing, 90%+ units condense flue gas and need a condensate drain.

Heat pump balance point is the outdoor temperature where heat pump capacity equals building load; below it, auxiliary or emergency heat supplements.

Defrost cycle on a heat pump reverses the refrigerant flow to melt frost on the outdoor coil; the auxiliary heat runs during defrost to temper supply air.

System performance checks: verify temperature rise across the furnace (typically 35–65°F), measure static pressure, and confirm refrigerant superheat/subcooling.

Combustion safety: measure CO in flue gas and ambient air; a CO reading above 100 ppm in the flue or any measurable ambient CO indicates a problem.

Heat exchanger cracks cause CO leakage and flame disturbance; a visual inspection plus combustion analysis is required before returning the unit to service.

Efficiency metrics: SEER2 and HSPF2 rate cooling and heating efficiency; higher values mean lower operating cost.